Computing applications typically rely on some type of storage medium for the persistent storage of various kinds of application data. For example, common office applications and multimedia applications generate and use application data of various types and formats, such as documents, spreadsheets, still images, audio and video data, among others. Frequently, such data is stored for repeated access or use on behalf of a user, or for shared use by multiple users such as employees of a particular department of a business organization. Users may wish to store and work with a number of documents or other data over a period of time, and may expect that the data will be readily available in a predictable state when needed. In many computing systems, the storage medium used by applications for persistent application data storage is most commonly a magnetic fixed drive or “hard” drive, although optical and solid-state storage devices are also used. Such devices are either integrated within a computer system that executes the applications or accessible to that system via a local peripheral interface or a network. Typically, devices that serve as application storage are managed by an operating system that manages device-level behavior to present a consistent storage interface, such as a file system interface, to various applications needing storage access.
This conventional model of application storage presents several limitations. First, it generally limits the accessibility of application data. For example, if application data is stored on the local hard drive of a particular computer system, it may be difficult to access by applications executing on other systems. Even if the data is stored on a network-accessible device, applications that execute on systems outside the immediate network may not be able to access that device. For example, for security reasons, enterprises commonly restrict access to their local area networks (LANs) such that systems external to the enterprise cannot access systems or resources within the enterprise. Thus, applications that execute on portable devices (e.g., notebook or handheld computers, personal digital assistants, mobile telephony devices, etc.) may experience difficulty accessing data that is persistently associated with fixed systems or networks.
The conventional application storage model also may fail to adequately ensure the reliability of stored data. For example, conventional operating systems typically store one copy of application data on one storage device by default, requiring a user or application to generate and manage its own copies of application data if data redundancy is desired. While individual storage devices or third-party software may provide some degree of redundancy, these features may not be consistently available to applications, as the storage resources available to applications may vary widely across application installations. The operating-system-mediated conventional storage model may also limit the cross-platform accessibility of data. For example, different operating systems may store data for the same application in different, incompatible formats, which may make it difficult for users of applications executing on one platform (e.g., operating system and underlying computer system hardware) to access data stored by applications executing on different platforms.
To address some of these limitations, in recent years some organizations have taken advantage of virtualization technologies and the falling costs of commodity hardware to set up large-scale network-accessible multi-tenant storage services for many customers with diverse needs, allowing various storage resources to be efficiently, reliably and securely shared by multiple customers. For example, virtualization technologies may allow a single physical storage device (such as a disk array) to be shared among multiple users by providing each user with one or more virtual storage devices hosted by the single physical storage device, providing each user with the illusion that they are the sole operators and administrators of a given hardware storage resource. Furthermore, some virtualization technologies may be capable of providing virtual resources that span two or more physical resources, such as a single large virtual storage device that spans multiple distinct physical devices.
For many applications and clients of such storage services, access requirements for a given storage object may change over time. For example, a document prepared for a meeting presentation may be accessed very frequently prior to the meeting (e.g., during various editing sessions by different collaborating authors), and for some time after the meeting, but the document may not be read or written very frequently six months after the meeting. Similarly, prior to the meeting, the impact on the customer of losing a particular version of the document as it is undergoing editing revisions may be much higher than after the meeting. Given that the pace at which data is generated for storage in such storage services has accelerated rapidly, the costs of supporting the highest-possible access performance, availability and/or durability for all storage objects regardless of the relative importance of the objects may eventually become prohibitive for at least some clients.
While embodiments are described herein by way of example for several embodiments and illustrative drawings, those skilled in the art will recognize that embodiments are not limited to the embodiments or drawings described. It should be understood, that the drawings and detailed description thereto are not intended to limit embodiments to the particular form disclosed, but on the contrary, the intention is to cover all modifications, equivalents and alternatives falling within the spirit and scope as defined by the appended claims. The headings used herein are for organizational purposes only and are not meant to be used to limit the scope of the description or the claims. As used throughout this application, the word “may” is used in a permissive sense (i.e., meaning having the potential to), rather than the mandatory sense (i.e., meaning must). Similarly, the words “include,” “including,” and “includes” mean including, but not limited to.